The invention relates to a driving stage for driving the two differential amplifier output stages of a subscriber line interface circuit, said driving stage being adapted to generate four driving currents in dependence on, on the one hand, transversal and longitudinal signals IT and IL, respectively, incoming to the driving stage and, on the other hand, a reference current IR, and to supply these driving currents in pairs to the two inputs of the differential amplifier output stages.
A purpose of a subscriber line interface circuit is, among others, to supply a subscriber line with current.
In a switching centre, the subscriber lines are in an idle condition during most of the time, i.e. the lines are open. Since the power losses in the subscriber line interface circuits constitute a considerable portion of the total power loss of the switching centre, it is very important to keep the power losses in the subscriber line interface circuits as low as possible in idle condition as well as during a call.
Normally, the output stages of analogue subscriber line interface circuits are of push-pull-type and operate in class-AB, which means that a certain idle current flows through the same.
Traditionally, the output stages of analog subscriber line interface circuits have been driven by means of current from two driving stages which normally operate between ground or +5V and battery voltage which nominally is -48V in connection with telephony. In order to obtain sufficient linearity in the driving of the output stages, the driving stages usually are designed to work in class-A which means that an idle current always will flow in the driving stages from ground or +5V to the battery voltage. This idle current has to be large enough in order to be able to drive the output stages to the maximum output current that is required, and is therefore dependent on the current amplification of the output stages.
Thus, relatively large currents which are independent of each other flow from ground or +5V to the battery voltage through driving stages and output stages also when the subscriber line interface circuit is not supplying any current. In this idle condition, it is of great importance that the power consumption of the subscriber line interface circuit is low. Considerable contributions are provided by the idle current of the driving stages as well as of the output stages.
Therefore, in the idle condition, the power loss caused by the driving stages and the output stages should be as low as possible.
Moreover, it is of great importance that the driving stages are able to drive the respective output stage across all of the available voltage range.